The Fairphone 6 is officially supported by the Installer.
After following the steps from the /e/OS (web)Installer, at my 2nd try, at the end I get the following error:
Error on step: connect-bootloader
Cannot execute command flashing unlock
Cannot execute command flash devcfg_a devcfg.img
error flashing file devcfg.img Bootloader emor: Bootloader replied with FAIL: Flashing is not allowed for Critical Partitions
At the 2nd try I got the bootloader screen that says: DEVICE STATE - unlocked , see screenshot
where the 1st try it was locked.
End at last I got the 2nd screenshot berfore it rebooted in Android 15